In electronic computers substantially all the electrical energy consumed by a computer is ultimately converted to heat. This heat must be removed from the equipment at a rate equal to the average rate at which electrical energy is converted into heat, otherwise the components of the computer will be destroyed. Furthermore, the heat removal must maintain the components of the computer within appropriate operating temperature limits to assure proper functionality. This problem is of significant concern in all computer systems. However, the problem of cooling is particularly acute in high speed, high capacity digital computers, referred to hereinafter as supercomputers, which operate with relatively high heat generating densities in the range of 275 watts per cubic inch, for example.
The capacity for cooling supercomputers has been enhanced as the result of the development of a variety of fluorocarbon (FC) cooling liquids, such as those manufactured by the Minnesota Mining and Manufacturing Company under the name Fluorinert (TM). These FC cooling liquids have high dielectric strength, low viscosity and relatively low boiling points, and are chemically inert. The relatively high dielectric strength of the FC cooling liquids allows the electronic components of the computer to be immersed or otherwise be placed in direct contact with the cooling liquid. One example of a prior art immersion cooling technique for a supercomputer which uses a FC cooling liquid is disclosed in U.S. Pat. No. 4,590,538. In this prior art system, the heat generating components of the supercomputer are directly immersed in a pool of FC cooling liquid. Heat is absorbed from the heat generating components by the FC cooling liquid, which is removed from the supercomputer and circulated through a heat exchanger to extract the heat.
The immersion type of prior art computer cooling system primarily transfers heat by using the sensible heat capacity of the cooling liquid. Sensible heat gain is heat gain which elevates the temperature of the cooling liquid or a cooling gas. The relatively low boiling points of the FC cooling liquids make them desirable for also using the latent heat capacity of vaporization to achieve cooling. Latent heat capacity is heat gain which results in a change in the phase state of the cooling liquid, when a liquid is changed to a vapor. The latent heat capacity of vaporization, i.e., that heat required to vaporize or boil a fixed amount of a liquid at its boiling point temperature, is much greater than the sensible heat capacity, i.e., that heat required to raise the temperature of the same fixed amount of the liquid by one degree. Although the cooling capacity could be further enhanced over that capacity available from immersion systems, a number of complicating factors have inhibited the effective use of heat gain from latent heat capacity in supercomputers.
A phenomenon known as thermal hysteresis can result in a temperature overshoot at the surface of a device being cooled at the incipience of boiling, in immersion cooling systems using the latent heat capacity of vaporization. In greatly simplified terms, thermal hysteresis can be understood as follows. When the boiling point of a liquid is reached vapor coalesces into bubbles which initially adhere in cavities on the surface being cooled. The bubbles will tend to remain in place until the forces which act to strip them from the surface, such as buoyancy and the flow of the liquid, are stronger than the forces which hold the bubbles in place. The bubbles tend to create localized areas on the surface that are not in contact with the liquid. In these localized areas the heat given up by the surface being cooled heats the vapor in the bubble to a temperature higher than the average temperature of the cooling liquid, a phenomenon known as superheating. A very high temperature exists at the localized areas on the surface to be cooled and, when averaged with those other areas in contact with liquid, elevates the average temperature of the surface being cooled. This creates the thermal hysteresis effect and results in a temperature overshoot at the surface. Thermal hysteresis can have a deleterious effect on the component being cooled because its temperature may exceed the nominal boiling point temperature of the surrounding liquid. Thus, even using a cooling liquid having a lower boiling point in an immersion does not assure that the temperature of the components will not reach destructive temperatures.
Cooling liquid flows in prior art immersion cooling systems have tended to adopt laminar, as opposed to turbulent, regimes. In a laminar flow regime, liquid can be visualized as flowing in several discrete planes or layers of flow with the plane immediately adjacent to a cooled surface, called the boundary layer, being stationary and the outer layers increasing in velocity as the distance from the cooled surface increases. There is resistance to the flow of heat from the cooled surface to the boundary layer and between layers. For the surface to be cooled, the temperature of the boundary layer must be higher than the layers farther from the surface. The boundary layer, and thus the cooled surface, thus experience a temperature significantly higher than the average temperature of the cooling liquid, thereby also inhibiting a more effective cooling effect.
A further complicating factor in cooling supercomputers is the extreme physical density of the logic elements. In order to take full advantage of the very high operating or clock speeds of supercomputers, it is necessary to minimize the propagation time of signals between logic elements. Large numbers of logic elements are typically grouped together in integrated circuits (ICs). One way to minimize signal propagation time is to create very short signal paths between the ICs. Such short signal paths result in a closely adjacent positioning and a dense assembly of ICs. In addition, the trend toward smaller ICs and toward ICs with a greater number of logic elements for the same size has resulted in an increased capacity for heat generation in a given volume. The decreased space between ICs and the greater integration of the ICs have not only contributed to increase the heat generation density but have also restricted the ability to achieve an adequate flow of the cooling liquid to the ICs. The collection of electrical conductors which supply electrical signals to the ICs also restricts the flow of cooling liquid. The collection of conductors, referred to as a "wire mat," may comprise up to seven hundred wires extending from each of three of the four sides of a four inch square circuit module upon which the ICs are positioned. To illustrate the magnitude of the heat generation density and cooling restriction factors, a supercomputer manufactured by the assignee of the present invention may have heat generation densities on the order of 275 watts per cubic inch, and total average heat generated on the order of 500,000 watts.
Several prior art techniques have been developed to partially overcome the problems of thermal hysteresis and boundary layer effects inherent in immersion cooling. Techniques which have been developed include jets or mists of cooling liquid spraying on the surface to be cooled, submerged jets of cooling liquid directed against submerged devices, submerged gas jets or bubble sources which impinge on submerged device to be cooled and falling films of cooling liquid directed over arrays of electronic devices. All of these prior art cooling techniques have resulted in improvement of heat transfer from the device to be cooled, but none of them appear to be capable of reliably delivering adequate cooling for a supercomputer which exhibits high physical placement density and high heat generation density of heat generating components.
It is against this background information that the present invention has evolved even further significant improvements and advancements in the field of cooling supercomputers, electronic components and other high heat density generating configurations.